1. Field of the Invention
The present invention relates to a signal conversion apparatus for converting an incoming signal by using a signal fed from a local oscillator. More particularly, the invention pertains to a technique for reducing an adverse influence of load variations on a local oscillator.
2. Description of the Related Art
FIG. 9 is a block diagram showing a basic configuration of a conventional frequency conversion apparatus provided with a local oscillator. The frequency conversion apparatus includes a mixer 102 which mixes an input signal having a frequency f1 and a signal having a specific frequency f3 output from a local oscillator 101 to produce two output signals having desired frequencies f2 equal to the sum and difference of the two input frequencies f1, f3 expressed by (f1+f3) and |f1−f3|.
In this kind of frequency conversion apparatus, if power of the input signal fed into the mixer 102 becomes higher than a specific value P, then the frequency f3 of the output signal of the local oscillator 101 varies with an increase in the power of the input signal. This phenomenon is known as “frequency pulling.”
FIG. 10 is a diagram showing how the frequency pulling phenomenon occurs in the conventional frequency conversion apparatus, in which a horizontal axis indicates the power of the input signal fed into the mixer 102 and a vertical axis indicates the frequency f2 of each output signal of the mixer 102. As can be seen from FIG. 10, the frequency f2 of the output signal of the mixer 102 is stable at a fixed value up to a point where the power of the input signal reaches the aforementioned specific value P. The frequency pulling phenomenon occurs beyond this point, causing the output signal of the mixer 102 to take the aforementioned two different frequencies f2 which progressively increase or decrease with an increase in the power of the input signal as illustrated.
FIG. 11 is a Smith chart indicating changes in load impedance viewed from the local oscillator 101 while the frequency pulling phenomenon is taking place. When the frequency pulling phenomenon occurs with the power of the input signal exceeding the specified value P, the load impedance viewed from the local oscillator 101 varies with an increase in the power of the input signal as shown by a solid arrow in FIG. 11. For this reason, the oscillating frequency f3 of the local oscillator 101 which is apt to be affected by load conditions varies with changes in the load impedance as shown by the arrow in FIG. 11.
As the frequency f3 of the signal generated by the local oscillator 101 varies in this fashion, the frequency f2 of each output signal of the mixer 102 varies and this causes degradation in functional performance of the frequency conversion apparatus.
The frequency pulling phenomenon occurs not only in the aforementioned frequency conversion apparatus but also in all kinds of signal conversion apparatuses provided with a local oscillator, such as a modulating apparatus for modulating a signal and a demodulating apparatus for demodulating a signal. One example of such signal conversion apparatuses is shown in FIG. 12 which is a block diagram showing a basic configuration of a conventional modulating apparatus provided with a local oscillator 101 and a modulator 202. In the example of FIG. 12, an input signal (modulating signal) of the modulating apparatus is a rectangular pulse wave having a frequency f1 and a signal (carrier signal) output from the local oscillator 101 is a sine wave having a single frequency f3.
In this modulating apparatus, a modulated signal output from the modulator 202 takes two states which are hereinafter referred to as “0” and “1” states for the sake of explanation. As the modulated signal alternates between the “0” and “1” states, load conditions (or the state of the modulator 202) viewed from the local oscillator 101 successively vary, thereby causing variations in the oscillating frequency f3 of the local oscillator 101 between the “0” and “1” states of the modulated signal.
As a consequence, the modulated signal of a frequency f2 output from the modulator 202 does not become an ideal burst wave but becomes a chirped wave, so that a transmitted radio wave produced by using the modulating apparatus will have a widened occupied bandwidth, for instance. This poses a problem that a narrow-band receiver used for receiving the radio wave transmitted by using the modulating apparatus of FIG. 12 can only achieve degraded receiving performance.
There exist conventionally known techniques for solving problems caused by frequency pulling. Given below are examples of such prior art techniques widely used to cope with the problems related to the frequency pulling phenomenon.
One example of this kind of technique disclosed in Japanese Patent Application Publication No. 1993-055828 is to prevent the occurrence of the frequency pulling phenomenon by suppressing load variations in an oscillator itself. Specifically, according to the technique of this Publication, the oscillator in which a signal generated by an oscillating circuit is output through a buffer circuit is configured such that a transistor of the oscillating circuit and a transistor of the buffer circuit are connected in cascade via an impedance converter made of an inductor, for instance. This configuration of Japanese Patent Application Publication No. 1993-055828 makes it possible to reduce load variations in the oscillator itself and thereby prevents the occurrence of the frequency pulling phenomenon.
The oscillator employing this configuration to prevent frequency pulling is however expensive compared to ordinary oscillators without such a configuration. In addition, it is possible to obtain a much higher degree of freedom in design by taking measures on an output signal of an oscillator to prevent frequency pulling rather than on the oscillator itself. Accordingly, measures aimed at preventing frequency pulling are mostly often taken on the output signal of an oscillator. Described below are examples of the measures taken on the oscillator output signal.
Among various approaches directed toward preventing frequency pulling, a first approach is to stabilize the frequency of the output signal of the local oscillator 101 based on a comparison with a frequency generated by a reference oscillator which is configured not to be affected by load conditions. The reference oscillator may employ a phase-locked loop (PLL), for instance, to stabilize its output frequency. Since the frequency of the output signal of the local oscillator 101 can be stabilized in this way, this first approach makes it possible to prevent the occurrence of the frequency pulling phenomenon by keeping the local oscillator 101 unaffected by the changes in load impedance shown in FIG. 11.
A second approach aimed at preventing frequency pulling is described in Japanese Patent Application Publication No. 2002-40130, for example. FIG. 13 is a block diagram of a conventional frequency conversion apparatus employing the second approach to preventing frequency pulling. Specifically, the second approach employed in the frequency conversion apparatus shown in FIG. 13 is to reduce the degree of coupling between a local oscillator 101 and a modulator 202 by inserting multiple stages of buffer amplifiers 103 therebetween. This configuration makes it possible to preclude load impedance viewed from the local oscillator 101 from changing and thus prevent the occurrence of the frequency pulling phenomenon. FIG. 14 is a Smith chart indicating this feature of the second approach employed in the frequency conversion apparatus of FIG. 13. As shown in FIG. 14, the load impedance viewed from the local oscillator 101 remains at a fixed point without changing with variations in power of the input signal.
While the aforementioned first approach makes it possible to prevent the occurrence of the frequency pulling phenomenon, it is necessary to perform phase locking operation. This develops such problems as greater complexity of circuit configuration of the local oscillator 101 and an increase in circuit scale.
The aforementioned second approach requiring a plurality of buffer amplifiers 103 inserted between the local oscillator 101 and the mixer 102 is associated with a problem that there is no way but to increase circuit scale up to a point where a desired frequency pulling characteristic is achieved. To obtain an optimum circuit layout which provides the desired frequency pulling characteristic, it is necessary to carry out complicated adjustment and evaluation procedures requiring considerable man-hours. The second approach has another problem that it is sometimes impossible to completely prevent the occurrence of the frequency pulling phenomenon in actuality.